The concentrations of tyrosine, tryptophan, and most other amino acids in the blood of humans and rats exhibit characteristic daily variations. We are exploring the mechanisms responsible for these rhythms, and the possible physiological consequences of time-dependent changes in blood and tissue amino acid levels. Our studies entail a) an examination of the rhythms in the activities of hepatic enzymes (e.g., tyrosine transaminase; tryptophan pyrrolase) that catabolize aromatic amino acids; b) determination of whether parallel rhythms exist in the rates at which amino acids in the systemic and portal circulations are catabolized; c) experiments designed to determine whether time-dependent increases in the brain levels of tryptophan or tyrosine cause parallel changes in the rates at which these compounds are utilized for the synthesis of neurotransmitters (serotonin; dopamine and norepinephrine) or brain peptides and proteins. Data obtained to date indicate that the amino acid rhythms in blood are largely the result of cyclic food consumption. These rhythms in blood are largely the result of cyclic food consumption. These rhythms cause parallel rhythms in, among other things, brain tryptophan content; this, in turn, accelerates the synthesis of serotonin. Projected studies include further examination of the consequences of amino acid rhythms in brain function. Chronic protein malnutrition causes abnormalities in the brain concentrations of monoamine neurotransmitters in rats, and in the excretion of neurotransmitter metabolites in humans. We are examining the biochemical mechanisms that cause these changes.